The present invention pertains to the process and equipment required for the cutting of the teeth prior to the installation of prosthetic crowns.
Usually, the preparation required before the installation of a crown on a tooth may be interpreted as consisting of the transformation of a spherical mass which is amputated at its base and at the top of a spherical portion, the base being sealed on a plane and the top remaining free, into a roughly straight tapered trunk whose large base coincides with the sealed base of the mass disposed thereabove.
Actually, in order to restore a tooth using a prosthetic crown, the approximately spherical mass of the tooth has to be transformed into an approximately straight tapered trunk. Thus, part of the peripheral dental surface is eliminated so as to be subsequently replaced by a restoring material.
The work involved on the approximately spherical mass, in order to change it into an approximately tapered trunk, leads to the definition of four vertical surfaces on the tooth. These surfaces are defined by the anatomical location of this organ within its environment, namely the buccal cavity, and by a free horizontal surface.
Each one of the four vertical surfaces is actually part of an approximately spherical surface of the approximately spherical mass. These four vertical surfaces of the approximately spherical mass are: the vestibular surface facing the cheek, the lingual or palatal surface facing the tongue, the distal surface facing the posterior tooth of the dental arch that includes the tooth considered, and the mesial surface facing the anterior tooth of the dental arch that includes the tooth considered.
Assuming that these four vertical surfaces each theoretically define a plane, the four respective planes so defined may be each considered to be parallel with the tooth axis. The tooth axis itself roughly defines the inserting axis of the future prosthetic crown.
Furthermore, the tooth includes an occlusal surface, which is the upper surface of the tooth. The occlusal surface is a free, horizontal surface defining a plane which is approximately perpendicular with the tooth axis.
A lower horizontal, sealed surface, representing the base of the rough spherical mass, is located at the junction between the portion of the tooth penetrating the gum with the bone supporting that tooth.
A line, traced along the approximately spherical mass and running at mid-height across the vestibular, lingual, distal and mesial surfaces, defines an intermediate plane which is approximately perpendicular with the tooth axis. The molar and premolar teeth are convex on all of their surfaces. The twelve teeth which belong to the incisivo-canine group are partially concave on their lingual surfaces, in the case of the lower teeth, and on their palatal face, in the case of the upper teeth. The intermediate plane thus defined divides the tooth into two anatomical zones.
The occlusal mass is located above the intermediate plane, and includes the occlusal surface of the approximately spherical mass. The cervical, or gingival mass is located below the intermediate plane, and includes the lower horizontal sealed surface of the approximately spherical mass. Together, the occlusal and cervical masses define the coronary mass which is an approximately spherical mass whose median axis is vertical.
The metal-type restoration techniques used by the contemporary crowns require strict specifications for the convergence of the sides and the limits of the peripheral cuts.
Despite the required accuracy, it is necessary to work by hand, using rotary-type tools, so as to perform the transformation into the approximately straight tapered trunk onto which the metal restoration of the dental organ has to fit with a minimum of friction. Actually, the mass to be transformed is not accessible to spinning or lathe-type machine tools. The tools that are used consist of drills in which a drill body is rigidly mounted with a handle which is rotatably driven by any suitable means. The machining processes currently used which include the cutting of the tooth without any preliminary rough shaping, do not guarantee the success of the operation as to the taper, and as to the location of the lower limit of the cut.